The invention pertains to dielectric resonators circuits. More particularly, the invention pertains to techniques for coupling energy to and from dielectric resonator circuits.
Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers and other circuits. The higher the dielectric constant of the dielectric material of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Suitable dielectric materials for fabricating dielectric resonators are available today with dielectric constants ranging from approximately 10 to approximately 150 (relative to air). These dielectric materials generally have a mu (magnetic constant) of 1, i.e., they are transparent to magnetic fields.
FIG. 1 is a perspective view of a typical dielectric resonator of the prior art. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. Individual resonators are commonly called xe2x80x9cpucksxe2x80x9d in the relevant trades. While dielectric resonators have many uses, their primary use is in connection with microwave communication systems.
As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system as determined by Maxwell""s equations. In a dielectric resonator, the fundamental resonant mode frequency, i.e., the lowest frequency, is the transverse electric field mode, TE01xcex4 (or TE hereafter). Typically, it is the fundamental TE mode that is the desired mode of the circuit or system in which the resonator is incorporated. The second mode is commonly termed the hybrid mode, H11xcex4 (or H11 hereafter). The H11 mode is excited from the dielectric resonator, but a considerable amount of electric field lies outside of the resonator and, therefore, is strongly affected by the cavity. The H11 mode is the result of an interaction of the dielectric resonator and the cavity within which it is positioned and has two polarizations. The H11 mode field is orthogonal to the TE mode field. There are additional higher order modes.
Typically, all of the modes other than the TE mode, are undesired and constitute interference. The H11 mode, however, often is the only interference mode of significant concern because it tends to be rather close in frequency to the TE mode. However, the TM01xcex4 or TM01 (Transverse Magnetic) mode also can be of concern. The longitudinal through hole 14 in the resonator helps to push the frequency of the Transverse Magnetic mode upwards. However, during the tuning of a filter, the frequency of the Transverse Magnetic mode could be brought downward and close to the operating band of the filter. The remaining higher order modes usually have substantial frequency separation from the TE mode and thus do not cause significant interference with operation of the system.
FIG. 2 is a perspective view of a microwave dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10. The resonators 10 are arranged in the cavity 22 of a conductive enclosure 24. The conductive enclosure 24 typically is rectangular, as shown in FIG. 2. Microwave energy is introduced into the cavity by a coupler 28 coupled to a cable, such as a coaxial cable. Conductive separating walls 32 separate the resonators from each other and block (partially or wholly) coupling between physically adjacent resonators 10. Particularly, irises 30 in walls 32 control the coupling between adjacent resonators 10. Walls without irises generally prevent any coupling between adjacent resonators separated by those walls. Walls with irises allow some coupling between adjacent resonators separated by those walls. By way of example, the field of resonator 10a couples to the field of resonator 10b through iris 30a, the field of resonator 10b further couples to the field of resonator 10c through iris 30b, and the field of resonator 10c further couples to the field of resonator 10d through iris 30c. Wall 32a, which does not have an iris, prevents the field of resonator 10a from coupling with physically adjacent resonator 10d on the other side of the wall 32a. 
One or more metal plates 42 are attached to a top cover plate (the top cover plate is not shown) generally coaxially with a corresponding resonator 10 to affect the field of the resonator to set the center frequency of the filter. Particularly, plate 42 may be mounted on a screw 43 passing through a threaded hole in the top cover plate (not shown) of enclosure 24. The screw may be rotated to vary the spacing between the plate 42 and the resonator 10 to adjust the center frequency of the resonator. The sizes of the resonator pucks 10, their relative spacing, the number of pucks, the size of the cavity 22, and the size of the irises 30 all need to be precisely controlled to set the desired center wavelength of the filter and the bandwidth of the filter.
An output coupler 40 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20 and into a coaxial connector (not shown). Signals also may be coupled into and out of a dielectric resonator circuit by other techniques, such as microstrips positioned on the bottom surface 44 of the enclosure 24 adjacent the resonators.
FIG. 3 shows one typical coupling element design that can be used as the input coupler 28 or output coupler 40 in the dielectric resonator circuit of FIG. 2. The resonator is shown at 31. The coupler 38 is mounted through the wall 32 of the resonator circuit and couples, for instance, to a coaxial cable 33 that carries a signal to or from the resonator circuit. The coupler 38 comprises a conductive loop 35 that is generally coaxial with and surrounds the dielectric resonator 31. The coupling loop can be an electric coupling loop or a magnetic coupling loop. Despite the terminology (which is conventional), coupling is predominantly magnetic in either case. Also, the coupling loop can be open or closed. If the loop is closed, the loop is fully coupled to the magnetic flux of the resonator. If the loop is open, it is only partially coupled to the magnetic flux of the resonator. For exemplary purposes, FIG. 3 shows an open, magnetic coupling loop that extends around the resonator 31 approximately 270xc2x0. An electric coupling loop, on the other hand, operates on the principal of capacitive coupling through a conductive plate positioned near the resonator.
Achieving a particular coupling strength between the loop and the resonator is crucial to meeting the desired filter specifications, especially return loss. Hence, selection of an appropriate type of coupling loop and appropriate selection of its other attributes, such as radius, position relative to the resonator and length of the wire, are essential to achieving such goals. One particularly significant attribute is the distance between the loop and the resonator 31. An adjusting screw 36 is mounted on the far side of the enclosure 37 opposite from the wall. In this particular design, there is another wall 39 of the enclosure 37 at that position and, thus, the adjusting screw 36 passes through and threadingly engages a hole 38 in the far wall 37. The adjusting screw 36 is nonconductive and can contact the loop 35 as shown in FIG. 3. By rotating the screw 36 so as to screw it into the cavity (to the left in FIG. 3), the distal end of the screw can contact the loop 35 and push it closer to the resonator, thus, increasing coupling. Likewise, by rotating the screw outwardly (to the right in FIG. 3), the loop can resiliently spring back out, thus moving further away from the resonator 31 and decreasing coupling strength.
As should be obvious, the adjusting screw 36 tends to deform the loop 35 so that it is not a perfect circle (or portion of a circle). This can cause coupling to be uneven, which is undesirable, and only has a fairly limited effect on the coupling strength between the loop and the resonator. Accordingly, the adjustment of the coupling strength by this technique is very limited and there is a need for an improved method and apparatus for adjusting the relative positions of a resonator and a coupling loop for tuning of the circuit.
Prior art resonators and the circuits made from them have many drawbacks. For instance, the volume and configuration of the conductive enclosure 24 substantially affects the operation of the system. Particularly, the enclosure minimizes radiative loss. However, it also has a substantial effect on the center frequency of the TE mode. Accordingly, not only must the enclosure be constructed of a conductive material, but it must be very precisely machined to achieve the desired center frequency performance, thus adding complexity and expense to the fabrication of the system.
Even further, prior art resonators have poor mode separation between the desired TE mode and the undesired TM01 and H11 modes.
Furthermore, as a result of the positions of the fields within the resonators, prior art resonators have limited ability to couple with microstrips, coupling loops, and other resonators. Thus, filters made from prior art resonators have limited bandwidth range. Further, prior art dielectric resonator circuits, such as the filter shown in FIG. 2, suffer from poor quality factor, Q, due to the presence of separating walls and coupling screws. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. The fields generated by the resonators touch all of the conductive components of the system, such as the enclosure 20, plates 42, internal walls 32 and 34, and adjusting screws 43, and inherently generate currents in those conductive elements. Those currents essentially comprise energy that is lost from the circuit.
The invention is a method and apparatus for coupling energy into or out of a dielectric resonator circuit by means of a coupling loop. More particularly, the invention is a method and apparatus for adjustably mounting a coupling loop relative to a resonator, the method and apparatus particularly adapted for use with conical and similar resonators in which the field of interest, typically the TE mode, varies as a function of longitudinal position relative to the resonator. In accordance with the invention, the coupling loop is supported from the distal end of a threaded screw that passes through a matingly threaded hole in the housing The resonator to which the loop is to couple is mounted on the distal end of a second threaded screw that passes through a matingly threaded central passage in the first screw. The position of the resonator, therefore, is longitudinally adjustable relative to the coupling loop by rotation of the second screw relative to the first screw. The resonator is longitudinally adjustable relative to the housing and the other resonators in the circuit by rotation of either the first screw or the second screw. By relative adjustment of the first and second screws to each other, the longitudinal position of the coupling loop relative to the resonator can be adjusted, thereby adjusting the coupling strength between the two. With this mounting technique, the coupling loop can be positioned very closely to the resonator to maximize field coupling. Furthermore, the coupling strength is adjustable by longitudinal adjustment of the coupling loop relative to the conical resonator.